1. Field of the Invention
The present invention relates to a single crystal wafer for a semiconductor laser, and more particularly, to a single crystal wafer for a semiconductor laser with an optimized cross section of a ridge of a cleavage surface composing an orientation flat.
2. Description of the Related Art
A semiconductor wafer has been used as a substrate for devices such as shot key gate field-effect transistor (MESFET), high mobility transistor (HEMT), hetero-junction bipolar transistor (HBT), laser diode (LD), and light emitting diode (LED), etc. An active layer of such devices is formed on a surface of a mirror surface wafer fabricated from a semiconductor wafer, by using Molecular Beam Epitaxy (MBE), MetalOrganic Vapor Phase Epitaxial Growth (MOVPE), or Ion implantation method.
For fabricating such a mirror surface wafer, at first, a crystal ingot is sliced with a predetermined thickness, and a wafer is obtained. Then, the sliced wafer is lapped by alumina abrasive grain of #800 to #3000 to remove a saw mark from the sliced wafer, thereby improving a surface smoothness. Thereafter, so-called “polishing” process, i.e. a process for finishing the wafer surface in a mirror surface by mechano-chemical polishing, is conducted by using a hypochlorous acid system aqueous solution or a mixture of hypochlorous acid aqueous solution and abrasive grain (silica, alumina, zirconium) as a polishing liquid, and a cloth having a porous layer on its surface is used as an abrasive cloth. Next, the finished mirror surface is washed by using a predetermined method and dried out. The dried mirror surface wafer is accommodated in a wafer tray or a wafer box.
On the other hand, when a compound semiconductor wafer is utilized for a semiconductor laser, the compound semiconductor may be provided with a cleavage surface as a reference surface, for the reason that an excellent smoothness of a resonant (lasing) plane is required, and that determination, alignment and focusing of crystal orientation are required. In particular, according to the specification of the compound semiconductor wafer product for semiconductor laser, an orientation flat part (OF part) or an index flat part (IF part) should be composed of a cleavage surface. The reason is as follows. In manufacturing a semiconductor laser diode, after forming an epitaxial layer on the wafer, it is necessary to cut the compound semiconductor wafer accurately along a cleavage surface to obtain a chip. Therefore, an angle adjustment should be conducted by using the OF part or the IF part composing of the cleavage surface as a reference.
Process of forming a cleavage surface of the OF part will be explained with referring to FIGS. 1A to 1D.
As shown in FIG. 1A, a single crystal ingot is sliced to provide a sliced wafer 1 having an OF part 1a and IF part 1b. Then, as shown in FIG. 1B, a short scratch (a notch) 2 is formed on a top surface or a back surface of the sliced wafer 1 by using a diamond pen, and a stress is applied thereto. A cleavage surface is obtained by scribing the sliced wafer 1 along a dotted line 3 (FIG. 1C). Thereafter, the sliced wafer 1 is chamfered such that the cleavage surface remains, and stepped portions on the cleavage surface due to the scribing cut is removed (FIG. 1D), to form an excellent cleavage surface (OF cleavage surface) 4, thereby providing a desired single crystal wafer 5 for a semiconductor laser.
FIG. 2 A shows a shape of a single crystal wafer for a semiconductor laser, and FIGS. 2B to 2E show an outline of a process for fabricating a semiconductor laser diode (LD) chip 6 using a single crystal wafer 5 for semiconductor laser fabricated as described above. Herein, a case of using a GaAs single crystal wafer is shown as a representative example. As shown in FIGS. 2B to 2E, this LD chip 6 is fabricated by a process comprising respective steps of (1) epitaxial growth, (2) formation of stripe structure, (3) formation of electrode, (4) formation of laser bar (facet coating of cleavage surface), (5) formation of chip (dicing), and (6) chip assembling.
In the semiconductor laser diode, an optical waveguide composing a resonator is formed within a semiconductor crystal. The optical waveguide has an elongated shape with a width of several micrometers and a length of several hundreds micrometers, and a reflection mirror is formed on both ends of the LD chip. In the GaAs single crystal wafer having a (100) plane as a surface, a longitudinal axis of the waveguide is formed in a direction perpendicular to the orientation flat (OF: <011> plane). The reflection mirror is formed automatically by “cleavage”, which is inherent in the III-V group compound semiconductor fabrication process.
As described above, when the semiconductor laser chip using the GaAs single crystal wafer is fabricated, a cleavage surface showing a crystal orientation [011] is used as a reference, and the chip is cut by cleaving in a direction perpendicular thereto, so as to employ the cleavage surface of the chip as a resonant plane. Therefore, it is important to obtain a parallelism between the cleavage surface and a mask pattern in formation of the mask pattern, so that the OF cleavage surface and the mask pattern are generally aligned by using a microscope.
In the conventional arts of forming the orientation flat (OF) itself, when the angle adjustment is conducted by using an optical method, e.g. by using the microscope, since a cross section of a ridge of the OF part is curved, it is difficult to focus a microscope observation image on the OF part in a state where the operator observes the OF part from a point of view beneath a mirror surface, so that a precision in angle adjustment is deteriorated. For facilitating the focusing on the OF part in alignment of the mask pattern, Japanese Patent Application Laid-Open (Kokai) No. 2000-068171 proposes a semiconductor wafer, in which an OF part is not chamfered and only side edges are processed by machining. Further, Japanese Patent Application Laid-Open (Kokai) NO. 2001-351836 proposes a semiconductor wafer, in which an OF part is formed by cylindrical grinding operation of a single crystal ingot, and an error in a crystal orientation of the OF part is measured thereafter, and the measured error is corrected by machining.
However, these prior arts do not refer to a relationship between the aforementioned “polishing” process and a “facet roll-off” formed at the OF cleavage surface.
Herein, the “facet roll-off” means a roundness (curvature) of a ridge of the OF cleavage surface 4 (a ridge angle made by a mirror surface 7 and the OF cleavage surface 4) as shown in FIG. 3B. A degree of the “facet roll-off” is expressed by a distance D from the mirror surface to a border between a curved part and a flat part, measured along the OF cleavage surface 4 (in a vertical direction).
According to investigation of an Inventor of the present invention, following disadvantage in the polishing process was found. A variation occurs in polishing conditions for finishing the chip surface into a mirror surface by “polishing” process. For example, if the polishing liquid is not provided around the surface of the wafer uniformly, a variation occurs in polishing rate on a plane of a wafer carrier plate, which functions as an affixing plate of the wafers. In addition, a processing pressure of the wafer varies in accordance with a variation of the number of wafers affixed on the wafer supporting plate. Such a variation of the polishing conditions causes the facet roll-off at an outer periphery of the wafer including the OF cleavage surface (D>40 μm, see FIGS. 3A and 3B), so that it becomes difficult to focus on the OF cleavage surface using the microscope in the alignment of the mask pattern, thereby deteriorating the precision in the alignment of the mask pattern.